Mixer Circuit; Metering Circuit - Icom IC-25A Instruction Manual

MIXER CIRCUIT
These modulated signals are taken out at the emitter of O6, and after amplified by |С2, are applied
to the same DBM that is used for the receiver.
127MHz band signals from the PLL unit, are mixed with the 16.9MHz modulated signals to give
signals of 127M Hz +16.9MHz. As a double balanced mixer is used for this mixing stage, 16.9MHz
and 127M Hz band signals are canceled and do not appear in the output.
DRIVER CIRCUIT
The signals are further passed through a helical cavity band-pass filter, to produce signals in the
144MHz band only, and then amplified by O7, O8, O9, and fed to PA module IC1 in the PA unit.
POWER AMPLIFIER CIRCUIT
The signals from Q9 in the MAIN unit are amplified by IC1 in the PA unit to obtain 25W output.
The heat from IC1 is transmitted to the diecast heat sink which radiates the heat very efficiently.
IC1 is a hybrid IC consisting of a two-stage RF power amplifier. The output signals of IC1 are fed.
to the antenna terminal through the T/R switching diode D1 and three-stage low-pass filter, which
suppress harmonics by more than 60dB.
ALC CIRCUIT
This circuit stabilizes the output power, even when the power voltage or the antenna load is
fluctuating, and switches the output power 1 watt and 25 watts. The variation in the current of
IC1 is detected at R1 in the PA unit and amplified by differential amplifier IC1 in the MAIN unit.
The output voltage from Pin 7 of IC1 is fed to O21 then O22. This lowers Q9's collector voltage
and input excitation level to the power amplifier module and reduces input power to the final
stage, thus preventing damage to the module due to high current. The output power can be ad-
justed by R124 in normal power (2bW), and by R116 in low power (1W) conditions.
RF POWER AMP CIRCUIT
METERING CIRCUIT
In the receive mode, the S/RF indicator functions as an S-meter. A part of the second IF signals
from 121 is rectified by D11 and 012 (for weak signals), and from Q13's collector is rectified by
D20 and D21 (for stronger signals). Both rectified signals are fed to the indicator driver IC, and
provide wide dynamic range meter function.
In the transmit mode, the S/RF indicator functions as a relative RF power meter. А part of the
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